1. Field of the Invention
This invention relates to a shaft seal which includes a sealing element formed of polytetrafluoroethylene or similar plastic material, and which is at least partially encased in an elastomeric sleeve for facilitating manual assembly of the seal and for improving the mounting and retention of the sealing element in the bore of a shaft seal housing.
2. Description of Prior Developments
The present invention has been primarily developed as an improvement of copending U.S. patent application Ser. No. 911,995, filed on Jul. 10, 1992. This prior application discloses a shaft seal that includes a one-piece annular seal element that has an annular mounting portion provided with an outer toothed surface insertable in a housing bore for retention of the seal element in the housing. An L-shaped sealing flange extends radially inwardly from the annular mounting portion for sealing contact with a rotary shaft extending through the shaft seal housing.
The seal element is typically formed of polytetrafluoroethylene ("PTFE") polymer, although other similar materials can also be used, e.g., urethane plastic, or a nitrile elastomer. The preferred material is PTFE because of its superior lubricity and low wear at elevated temperatures, and resistance to chemical attack by oils and other liquids.
However, PTFE has a relatively high coefficient of thermal expansion and a propensity to creep after prolonged exposure to thermal stress caused by elevated operating temperatures and the resulting applied pressure. Also, PTFE does not bond easily to metals or other materials. Additionally, the low coefficient of friction of PTFE makes it difficult to mount the material using a frictional clamping force. These factors make it difficult to securely mount and retain PTFE shaft seals in their casings as well as in their associated housing bores.
In the shaft seal disclosed in the above-mentioned U.S. patent application Ser. No. 911,995, the seal element is retained in the seal housing partly by a toothed surface on the seal element mounting portion and partly by the action of an annular metal band engaged with the inner annular surface of the seal element mounting portion. The metal band assumes a rigid configuration within the annular plastic mounting portion so that, when the seal assembly is forced into the seal housing bore, the plastic material is squeezed between the bore surface and the rigid metal band. The rigid metal band counteracts the tendency of the plastic material to shrink or creep after prolonged exposure to elevated temperature and pressure. Such creep or shrinkage, if unopposed, would allow the plastic seal element to become loosened in the housing bore and possibly fall out of the bore.
In the above-described seal element construction, the metal reinforcement band is retained on the inner surface of the plastic sealing element mounting portion by an annular plastic lip extending from the plastic sealing material radially inwardly along an edge of the band. The band is installed within the sealing element by moving the band axially across the lip and into the confined space on the inner surface of the seal mounting portion.
The rigidity of the plastic lip material is such that the operation of inserting the metal band into the sealing element must be performed with a press or similar machine. It would be desirable to be able to install the reinforcement band into the sealing element by a manual hand operation. However, hand insertion of the metal band into the plastic sealing element is typically not possible with the arrangement shown in the above-mentioned U.S. patent application Ser. No. 911,995.
Accordingly, a need exists for a shaft seal construction which allows easy hand installation of an elastomeric sleeve and a metal support band within a machined seal assembly of the type noted above.